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Darwin Correspondence Project

From Henry Denny   23 January 1865

Phill Hall Leeds

Jan 23d/65

Dr Sir

With regard to Mr Marshalls statement respecting the Polynesian Lice,1 it is a very extraordinary statement, & I cannot see any reason why the Lice of one Human being should not live upon another! It is a fact that the parasites of one Genus of animals do not live upon animals of another Genus. Swine upon the Horse or Ass or Ass upon the Ox—or Horse, Deer   The same with Birds. Species of the same Genus as the various Ducks have frequently the same Louse   again some of the smaller Passerine Birds. But the Goose & Swan have not the same Louse or that of the Duck or Merganser. As to the identity of the same pediculus on different Races of Men—this is a question of some perplexity. And the first question to decide is are the different races of men varieties only of one Genus or Species! I presume the former, & if so, the differences which exist & are constant in the characters of the Lice of these varieties, are not greater than we find in the races themselves, or in the varieties of the Genus Canis, or Columba, or Gallus. Yet the same species of Lice infest the different varieties of Fowl, Dogs & Pigeons.2 Mr Andrew Murray has paid more attention to this subject, than anyone else in Britain whose paper I send you, from which you will perceive, there are well marked characters which distinguish the anterior legs & antennæ of the Lice of different Races as also the colour—, but as he truly observes, these are not greater, than those of the Races themselves ∴ if we cannot believe that they are any thing more than varieties, the same will apply to the Parasites, which also remain fixed!3 I think it very probable Mr Marshalls statement as to the darker colour & difference in appearance: to a general observer, may be correct enough!4 The questionable point is their not living upon the English Sailors!. (Dr Milligan,5 who resided many years in Tasmania informed me that the aborigines of Tasmania stated to him, that until introduced by Europeans they had never seen the Louse on the Human body! & he had no reason to doubt their veracity! The natives wear no clothes whatever & the women shaved their heads &c— & the men although they wore long hair, so besmeared it with fat & red ochre, as to render it altogether unfit for the abode of even a Louse.

Mr Murray describes two varieties of Australian Lice, which differ in colour &c from the European. 6 are we to suppose the Australian Lice are an Introduced from some other race? or aboriginal. if the latter, It is difficult to believe the Tasmanians have been excepted from Parasites. I should rather have suspected the smearing of the Hair was to destroy or extirpate them! and if the women shaved the hair this very practice was adopted by the Egyptian Priests to keep themselves free from vermin!— I believe the Human races in every part of the world have Lice, which may offer slight modifications of character! But I would not commit myself to decide, whether we shall call these specific characters or not. The transition between species in some instances is so gradual that it becomes difficult to say where the variety or Species, begin or terminate!— I fear you will not obtain much information from this rambling note, but any grain is quite at your service.—

Believe me | Dr Sir | Yours very truly | Henry Denny

Charles Darwin Esq | &c—

P.S. I shall be glad of Mr Murrays Paper back again when you have done with it.

CD annotations

0.3 Dr Sir … perplexity 1.9] crossed red and blue crayon
1.13 Yet the … & Pigeons. 1.14] scored red crayon; square brackets around text, red crayon
1.19 they are … with it. 5.1] crossed red crayon


See letter to Henry Denny, 17 January [1865] and n. 5. CD had sought Denny’s opinion on a statement made by Marshall, a surgeon on a British whaling vessel whom CD had met during the Beagle voyage. Marshall claimed to have observed that lice that normally lived on the bodies of Sandwich Islanders were unable to survive on the bodies of the English.
In Descent 1: 219, CD mentioned on Denny’s authority that the same species of lice (Pediculus) were found on different varieties of dogs, fowls, and pigeons. See CD’s second annotation to this letter.
See Murray 1860, pp. 571–3. Andrew Murray’s paper addressed the argument that each human race possessing a distinct parasite must be a distinct species. CD cited Murray’s paper in Descent 1: 219–20, remarking that in insects even slight structural differences, when constant, were generally regarded as of specific value, and adding (p. 220): ‘the fact of the races of man being infested by parasites, which appear to be specifically distinct, might fairly be urged as an argument that the races themselves ought to be classed as distinct species’. CD went on to discuss other evidence bearing on the unity of the human species, noting that the problem was ‘a hopeless endeavour’ until some definition of the term ‘species’ was generally accepted, and suggesting ultimately that the category of ‘sub-species’ was ‘most appropriate’ (Descent 1: 228, 235). CD also supported the view that human races had descended from a common stock (ibid., 2: 388).
See R. D. Keynes ed. 2000, p. 283, and letter to Henry Denny, 17 January [1865] and n. 5. Marshall’s description of the lice appears in Descent 1: 219–20.
Joseph Milligan.
Murray 1860, p. 576.


Descent: The descent of man, and selection in relation to sex. By Charles Darwin. 2 vols. London: John Murray. 1871.


Species of lice and the animals they infest. Different kinds of dogs, fowls, and pigeons are infested by the same species of Pediculi [see Descent 1: 219].

Letter details

Letter no.
Henry Denny
Charles Robert Darwin
Sent from
Source of text
DAR 80: B150–1
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 4753,” accessed on 28 October 2021,

Also published in The Correspondence of Charles Darwin, vol. 13